1. Field of the Invention
The present invention relates to image forming apparatuses such as electrophotographic printers, copy machines and facsimile machines, and more particularly to a registration adjustment method for an image forming apparatus that forms and prints color images and such an image forming apparatus.
2. Description of the Related Art
With a conventional image forming apparatus, latent images are written onto a plurality of respective image carriers (photosensitive drums) with laser light from an optical scanning device (LSU). The latent image on each image carrier is developed with toner of the corresponding color so as to form a visible image of the color on the image carrier. The visible images of respective colors are transferred, one above the other, from the image carriers to an intermediate transfer member (intermediate transfer belt), so as to form a color visible image on the intermediate transfer member. Also, the color visible image is transferred from the intermediate transfer member onto recording paper. With such a conventional configuration, there are cases where color drift occurs in which visible images of respective colors are displaced with each other when transferred from the image carriers to the intermediate transfer member, resulting in degradation of the quality of the color image. To address this, displacement of a visible image of each color is corrected by measuring the amount of displacement (in other words, registration adjustment value) of the color visible image on the intermediate transfer member, and controlling the write timing of a latent image onto the corresponding one of the image carriers, thereby shifting the position of the latent image on the image carrier by an amount corresponding to the registration adjustment value.
However, due to change in the temperature (ambient temperature or internal temperature) of the optical scanning device (LSU), the position of the latent image on each image carrier varies, and the amount of displacement of the visible image of each color on the intermediate transfer member varies. Accordingly, displacement of the visible image of each color cannot be accurately corrected by keeping the registration adjustment value fixed.
In view of this, for example, JP 2007-86439A (Patent Document 1) discloses an image forming apparatus in which a temperature sensor is provided near the optical scanning device (LSU), the registration adjustment value is corrected according to the change of the ambient temperature detected by the temperature sensor, whereby displacement of the visible image of each color is corrected more accurately.
Also, JP 2009-63650A (Patent Document 2) discloses an image forming apparatus in which a first temperature sensor for detecting the temperature between a paper discharge tray and a first scanner and a second temperature sensor for detecting the temperature between the first scanner and a second scanner are provided, and color drift correction is performed by adjusting the lasing position based on the temperatures detected by the first and second temperature sensors.
Furthermore, JP 2004-101889A (Patent Document 3) discloses an image forming apparatus in which an imaging means, an endless belt, a first temperature sensor for detecting the temperature near a transfer means or the like, and a second temperature sensor for detecting the internal temperature of the image forming apparatus are provided, and color drift correction is performed by determining the operation timing of a color drift correction means based on the temperatures detected by the first and second temperature sensors.
The main cause of color drift is considered to result from the optical scanning device (LSU), in which when there is a change in the internal temperature of the LSU, the orientation of each mirror provided in the LSU changes.
Accordingly, it is considered reasonable to correct color drift by using a method in which the internal temperature of the LSU is detected, and the registration adjustment value is corrected according to the change of the internal temperature. Also, inside the LSU, because a driving motor for a polygon mirror generates the largest amount of heat, detecting the temperature near the polygon mirror is considered essential.
However, Patent Document 1 detects the temperature near the LSU, rather than the internal temperature of the LSU. Also, Patent Documents 2 and 3 detect temperature at a plurality of locations, but they detect the ambient temperature outside the LSU, rather than the internal temperature of the LSU. Since the change of the internal temperature of the LSU, which is the main cause of color drift, is not referred to, it is difficult to improve the accuracy of color drift correction.
The experiments performed by the inventors of the present application have revealed that merely correcting the registration adjustment value according to the temperature change near the polygon mirror does not result in accurate correction of displacement of the visible image of each color, and there is a possibility that displacement of the visible image of each color might become larger.
For example, when the internal ambient temperature of the image forming apparatus changes from low to high immediately after activation of the image forming apparatus, or when the image forming apparatus has transitioned from continuous printing state (full-capacity operation state) to intermittent printing state, and the internal ambient temperature of the image forming apparatus changes from high to low, the ambient temperature starts to change much later than the temperature change near the polygon mirror, so the registration adjustment value cannot be accurately corrected only by correcting the registration adjustment value according to the temperature change near the polygon mirror until the ambient temperature starts to change, and registration obtained as a result of correction degraded than before correction.
Accordingly, when the LSU is the main cause of color drift, it is essential to correct the registration adjustment value according to the temperature change near the polygon mirror, and at the same time, in order to improve the accuracy of color drift correction, it is also necessary to give consideration to the internal ambient temperature of the image forming apparatus.